<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>DOM</title>
</head>
<body>
    <button id="btn">click</button>

    <script>

    // document.getElementById('btn').onclick = function () {  
    //     alert("hello");
    // }


    /* 
    getElementsByTagName获得的是数组 要遍历才能绑定 */
    document.getElementsByTagName('button').onclick = function () {  
              alert("hi");
    }

    console.log(document.getElementsByTagName('button'));

    </script>
</body>
</html>